The Condor 91:362-371 0 The CooperOrnithological Society 1989 HAWK OCCUPANCY OF DISTURBED GRASSLANDS IN RELATION TO MODELS OF HABITAT SELECTION ’ JOSEF K. SCHMUTZ Department of Biology, Universityof Saskatchewan,Saskatoon, SK S7N 0 WO, Canada Abstract. I counted nesting pairs of FerruginousHawks (Buteo regalis)and Swainson’s Hawks (B. swainsoni)on 76 41-km2 study plots in 1982 and 1987 to study the effect of agricultural cultivation of grasslandon hawk densities. The study plots were selected at random within a 74,686-km2 mixed-grassprairie region in southeasternAlberta. The results confirm a curvilinear relationship between hawk density and extent of cultivation. Nesting densitiesof both specieswere higher in areasof moderate cultivation than in grassland.On plots with extensive cultivation (>30%), Ferruginous Hawks declined and so did ground squirrels,their main prey. Swainson’s Hawks did not decline with extensive cultivation but probably shifted to other prey where ground squirrelswere scarce.The observedcurvilinear relationship between hawk density and habitat is similar in shape to changesin the pro- ductivity of ecosystemssubjected to varying degreesof disturbance. The pattern of hawk abundance in relation to habitat quality did not change between years despite a more than 50% increase in hawk densities. The hawks therefore did not conform to the general assumptionin models of habitat selectionthat optimal habitats are saturatedbefore suboptimal areas are occupied. Instead, new breeding pairs continued to settle in optimal habitat but not suboptimal habitat. To account for these results,I propose a graphical model which incorporatesterritoriality as a factor causingregular dispersion at medium population densitiesacross the rangeof habitat typesincluded in this study.Optimal habitat may be the last to reach saturation becauseonly there can territory size be reduced furthest under increasingintruder pressure. Key words: FerruginousHawk; Buteo regalis; Swainsons’ Hawk; Buteo swainsoni;Al- berta; habitat-selectionmodels: grassland cultivation; population dynamics; territoriality. INTRODUCTION and individuals disperse (“free distribution”), The large number of published reports that de- such that in the end they experience relatively scribe habitat selection among birds attests to equal feeding opportunity in sites varying in re- the enormous variation observed and to the bi- source availability. Given differences in suita- ological importance of this topic. In addition to bility between habitats, the prediction is that areas its basic biological importance, an understanding of lower suitability are occupied only after the of the responses by birds, and other animals, to suitability of the most optimal areas has been changes in habitat characteristics is important for reduced by the need to share resources with oth- conservation. Habitat degradation emerges as a ers. The model predicts that members of a species major threat to conservation in many countries. for which this model applies experience rela- Functional and theoretical aspects of habitat tively equal habitat suitability and hence equal selection have been summarized by Cody (1985). fitness for that component of fitness that is de- Fretwell and Lucas (1970) and Fretwell (1972) rived from habitat suitability. provide a theoretical framework for habitat se- Species that exhibit territoriality such that their lection. Their model illustrates the choices in- density can be greatly dependent on this behavior dividuals are forced to make when attempting to (e.g., Brown 1969) violate the “free” assumption maximize fitness in habitats varying in suitabil- of the ideal free distribution model. To include ity for optimal survival and reproduction. They the effect of dominance and territoriality, Fret- postulate first that organisms are able to judge well (1972) incorporated a risk accruing to un- an area’s basic suitability and seek those areas settled individuals in the “ideal despotic distri- best suited (“ideal distribution”). Second, under bution model.” Because territories are generally increasing intruder pressure suitability decreases smaller and hence more easily defended in the most suitable areas, territoriality can create dif- 1Received 27 September 1988. Final acceptance30 ferences in fitness between individuals in areas January 1989. differing in their basic suitability. [36-V HABITAT SELECTION BY PRAIRIE HAWKS 363 In this analysis, I report on a “natural exper- lected at random within a 74,686-km* survey iment” in which the habitat selection of Ferru- region in southeasternAlberta. This region was ginous and Swainson’s hawks was studied twice, bounded by Saskatchewanand Montana. Within 5 years apart, on randomly selected study plots Alberta, the study area included semi-arid, in southeastern Alberta. During the 5-year in- mixed-grass prairie habitat located south of terim, the abundance of Richardson’s ground 5 l”42’N latitude and east of 114”OO’W longitude. squirrels (Spermophilus richardsonii) has in- I searchedplots for nests in 2 years. In 1982, I creasedmarkedly (J. K. Schmutz and D. J. Hun- searched80 plots (Schmutz 1984) and in 1987, gle, unpubl. data), and so has the population den- 78 plots, 76 of which were searchedduring both sity ofboth hawks (Schmutz, unpubl.). The study years. plots used were the same in both years. Changes I searchedstudy plots completely for all nests in the degree of cultivation between years were of buteonine hawks. The primary mode of trans- small. In a within-year comparison, I examine portation on plots was by motorcycle but some differences in the densities of breeding hawks sites were searchedusing a truck or on foot. In between plots varying in degree of cultivation. I most areas, nests were found easily in the gently also examine habitat selection between years undulating landscapewith few trees. In addition when the overall population size of hawks dif- to nestscontaining eggsor young, some deserted fered. The observed patterns of habitat selection nests (4.7% of all nests) that showed evidence of are compared with predictions based on models use during the present year (Postupalsky 1974) of habitat selection. I make the fundamental as- were counted. In these cases,species was deter- sumption in this analysis that individuals on dif- mined by the type of nest material (Schmutz et ferent plots are drawn at random from the pop- al. 1980) and color offeatherspresent. The 6-week ulation pool and that they do not represent searching period, from late May to early July, different ecotypes. included the hatching and nestling periods of The Ferruginous and Swainson’s hawks stud- Ferruginous Hawks, and the latter part of laying, ied exhibit intraspecific territoriality (Schmutz et all of incubation, and the early nestling period al. 1980, unpubl.). Their habitat selection has of Swainson’s Hawks. been described (Olendorff 1973; Schmutz et al. Steeply eroded banks existed on the study area 1980; Bechard 1982; Gilmer and Stewart 1983, along rivers and creeks. Some grasslands had 1984; Schmutz 1984, 1987; Janes 1985). Unlike never been tilled, others had undergone second- Swainson’s Hawks, Ferruginous Hawks are high- ary successionfrom fields abandoned during the ly selective in their choice of grassland habitat 1930s (Dormaar and Smoliak 1985). The pri- for nesting. This habitat dependence is so pro- mary land useswere grazing of cattle and growing nounced that an availability of grasslandhas been of cereal grain, depending on soil quality for ag- considered important in limiting this species’ riculture and nature of terrain. In 1982 the av- breeding distribution. erage percent cultivation was 50% and in 1987 The present study differs from most others (cf. it was 53%. Cody 1985) in some potentially important ways. For each plot, the locations of nests, fields, First, the speciesstudied are large raptors which, pastures, trees, and water bodies were recorded as adults at least, are threatened by few if any on a data sheet at the time of searching.I deter- heterospecificpredators. Second, density is that mined the percentageof land area that was cul- of breeders only. Third, comparisons of hawk tivated from thesemaps for each year separately. density are made within a grasslandhabitat that Fields that had been reseededwith “tame” grass varies in degreeof disturbance due to agricultural varieties and used for grazing were recorded as cultivation. Thus, this study examines responses grassland. Fields of clover, that were irrigated by the hawks to alterations (cultivation) of one and used for forage production, were recorded type of habitat (mixed-grass paririe) and not dif- as cultivated becauseof their similarity to cereal ferencesbetween two natural habitats. crops in vegetation height and density. I recorded the number and activity of hawks presentwhen their nest was found. Nest contents STUDY AREA AND METHODS were inspectedwhen accessiblewithout climbing Nesting pairs of Ferruginous and Swainson’s aids. In late May and early June, clutches of hawks were counted on 4 1-km2 study plots se- Swainson’s Hawks were considered complete if 364 JOSEF K. SCHMUTZ down feathers were present in the nest cup and ously (Schmutz et al. 1980, 1984). To minimize the incubating hawk flushed at a distance of 20 the confounding influence of tree availability on m or less. Because the nests were visited only hawk density, I excluded plots which bore no once between the latter period of laying (Swain- trees. This approach was slightly conservative